Optimum Design Of Double-row Pile Supporting Structure For Deep Foundation Pit Near Metro Tunnel

With the development of society and the acceleration of urbanization,the issue of deep foundation pit support has become a new hot spot and difficulty in civil engineering construction.The deep foundation pit supporting structure of underground structures such as subway tunnels and subway stations is constrained by the nearby soil,which affects the deformation and stability of the supporting structure,which causes the distribution of earth pressure to be different from the normal conditions.Based on the research on this kind of problem,there are still relatively few studies.This paper draws on the research methods of the predecessors,taking the deep foundation pit supporting engineering of a nearby subway tunnel in Xi an as the background,considering the subway tunnel,deep foundation pit and support.The interaction between the structures was simulated by the finite element calculation software ABAQUS.The deformation law of the deep foundation pit double-row pile retaining structure under different parameters was discussed and the following conclusions were obtained:(1)The length of the pile,the diameter of the pile,the stiffness of the pile,the spacing of the piles and the change of the pile spacing can all change the deformation characteristics and the bearing performance of the double-row pile retaining structure.In addition,the crown beam affects the displacement of the pile top of the double-row pile retaining structure,and the pile length and pile spacing affect the displacement of the pile bottom.(2)In terms of the degree of influence and the priority of construction,the row spacing has the most obvious influence on the deformation behavior of the double-row pile retaining structure.The size of the row spacing directly determines the distribution ratio of the earth pressure between the front and rear rows of the double-row piles,which leads to the difference in the deformation characteristics of the piles in the front and rear rows of the double-row piles.In addition,the influence of pile spacing and pile stiffness on the double-row pile retaining structure is second,and the pile length and pile diameter have the least influence on the double-row pile retaining structure.(3)According to the different bending moment distribution characteristics of double-row piles,different reinforcement methods can be adopted:when the distribution of bending moments of single piles is different,segmental reinforcement can be used;when the bending moments of the current rear piles are different,it can be adopted.Asymmetric reinforcement;when the increment of the bending moment of a single pile is different,it can be used in combination with the segmentation and asymmetric reinforcement.(4)The smaller the distance from the foundation pit to the tunnel,the more obvious the impact of foundation pit excavation on the subway tunnel.When the distance from the foundation pit to the tunnel becomes larger,the existence of the subway tunnel can greatly reduce the horizontal displacement of the pile,change the deformation and deformation performance of the double-row pile,and the surface settlement between the foundation pit and the tunnel will be reduced.However,the amount of surface settlement away from the foundation pit will continue to increase.As the distance from the base tunnel increases,this effect will gradually weaken until it completely disappears.When the distance from the foundation pit to the tunnel increases from 10m to 30m,the surface settlement near the foundation pit will increase by 5-6 times.(5)The deeper the depth of foundation pit excavation,the greater the impact of foundation pit excavation on the subway tunnel.The existence of the subway tunnel has a great impact on the foundation pit excavation project adjacent to it.The existence of the subway tunnel can significantly change the settlement mode of the soil between the foundation pit and the subway tunnel and the deformation characteristics of the double-row pile retaining structure.With the increase of the excavation depth of the foundation pit,the influence of the subway tunnel on the foundation pit excavation will be gradually weakened,the horizontal displacement generated by the double-row pile retaining structure will be greatly increased,and the surface settlement around the foundation pit will also increase rapidly.When the depth of the foundation pit is increased from 10m to 25m,the maximum settlement of the ground around the foundation pit is also four times as large as the original.(6)The settlement form of the back soil in the east side of the deep foundation pit near the subway tunnel is groove-shaped settlement,while the settlement characteristics on the west side of the foundation pit are different:near the foundation pit,the surface appears concave The trough is settling,but the settlement is much smaller than the east side of the foundation pit;at a distance from the foundation pit,the surface settlement gradually increases with the distance from the boundary of the foundation pit.